Generally, the reproduction characteristic of a copying apparatus has a three-part form, namely a first part which without distinction maps low optical densities on the original to the color white on the copy, a second part which with distinction maps grey tints to optical densities between white and black, and a third part which without distinction maps high optical densities, i.e., dark grey tints, on the original to the color black on the copy. The color black corresponds to the maximum optical density that can be supplied by the printing system.
Documents may differ considerably in the number of grey tints present, and it is frequently desirable to be able to optimize the reproduction of the grey tints by a copying apparatus for the type of document that has to be copied at any particular time. This can generally be achieved by using an adapted reproduction characteristic. In analogue copy apparatus (which makes a copy by charging a photoconductive medium, exposing imagewise with a projection of the document for copying, and developing the residual charge image with toner powder), it is possible to adapt the reproduction characteristic by adjusting the quantity of light used to make the projection. The reproduction characteristic shifts as a result and specific grey tints on the document which are reproduced without distinction under normal exposure, e.g. the dark tints, are now separately visible. This is usually at the expense of the distinctive reproduction of other grey tints, the lighter ones in this example.
With the advent of the digital copying machine it has become possible to treat grey tints as digital data and process them as such, with much less dependence on other grey tints. The present invention is aimed at this area of the art.
A method and apparatus having an adjustable mapping of optical densities are known from Paul M. Butterfield, "User-Oriented approach to control of tone reproduction for electronic reprographic systems", SPIE Vol. 1670, Color Hard Copy and Graphic Arts (1992), p.512-520. This document describes how a reproduction characteristic in the optical density domain can be adjusted by an operator. The reproduction characteristic is a graph indicating the relationship between input optical density ODin (on the X-axis) and output optical density ODout (on the Y-axis). According to Butterfield, a value on the Y-axis, i.e. an ODout value, can be adjusted by an operator at five approximately equidistant points on the X-axis. These ODout values are interconnected by straight lines and thus determine the reproduction characteristic. By determining five points an operator can define a complicated reproduction characteristic and hence very accurately control the reproduction of a document having a large number of grey tints.
The known method offers an operator very great freedom for programming the reproduction characteristic, but consequently requires a large number of adjustment operations, the effect of which cannot always be completely seen.